According to a preferred embodiment of the invention one or more tiltable closure flaps are provided at the upper end of the flue gas ducts for the connection or disconnection of the one flue gas duct or group of flue gas ducts. Such a closure flap may be situated between the flue gas ducts and serves alternatively for complete or partial closure of one or the other of the ducts. Preferably a central position of the closure flap may be set in which all the groups of flue gas ducts are opened for the flue gases to flow therethrough. In this setting, due to the increase of the surface available for the heat exchange, a higher energy output is obtained when firing with liquid fuel, because the hot gases flow through the clean ducts for firing with liquid fuel and simultaneously also through the ducts for firing with solid fuel which may be covered with soot.
In order to adapt the heating output of the boiler even better to requirements due to specific circumstances, the closure flap preferably incorporates a plurality of individual flaps by means of which the ducts may be covered either fully or partly. The closure flap is preferably controllable from outside the boiler. Particularly according to a preferred embodiment of the invention a control mechanism is provided for the closure flaps by means of which the setting of the closure flaps is controllable according to the sensed temperature of the outlet gases. This ensures that when the temperature of the outlet gases is low the flap may be fully or nearly fully closed and when it is high it may be opened in order to utilize energy economically.
According to a further embodiment of the invention the ducts are formed by two or more mutually parallel fire tubes. In order to act as heat exchangers the fire tubes are surrounded by a jacket for the flow of water to be heated. The advantage of the use of fire tubes is that they can be easily and reliably cleaned by a round brush so that after cleaning the whole wall surface is again available for maximum energy transfer.
According to a further embodiment of the invention the ducts may be formed instead by a plurality of spaced apart mutually parallel wall surfaces which are made as water heating pockets for the flow of water to be heated therethrough. When the ducts are made in this way a particularly large wall surface is available for the exchange of heat. The disadvantage of ducts made in this way is that their cleaning is difficult. It must be kept in mind that the cleaning of corners and edges cannot be perfect and these uncleaned areas contribute little to heat transfer.